<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="../../js/vue.js"></script>
</head>

<body>
    <div id="root">
        姓：<input type="text" v-model="firstName"><br><br>
        名：<input type="text" v-model="lastName"><br><br>
        全名：<span>{{fullName}}</span>
    </div>
    <script>
        Vue.config.productionTip = false //取消生产提示

        const vm = new Vue({
            el: '#root',
            data() {
                return {
                    firstName: '张',
                    lastName: '三',
                    fullName: '张-三'
                }
            },
            watch: {
                firstName(newValue) {
                    setTimeout(() => {
                        this.fullName = newValue + '-' + this.lastName
                        console.log(this);
                    }, 1000)
                },
                lastName(newValue) {
                    this.fullName = this.firstName + '-' + newValue
                }
            }
        })
    </script>
</body>

</html>